The in vivo serological detection of H-Y (male) antigen on the surface of mouse spermatozoa and male epidermal cells, with the cytotoxic test, provides the means for a closer evaluation of humoral versus cellular immune factors involved in H-Y incompatibility, and of other questions about the H-Y antigenic system. Our aim is to: (1) Determine to what extent there is polymorphism for H-Y within the species (mouse), (2) Gain further insight into the mechanism involved in the prolonged survival of male grafts on certain strains of mice which are capable of producing antibody against the H-Y antigen, (3) Determine the H-Y immune status of multiparous C57BL/6 female mice which retain male skin grafts, and (4) Determine whether or not antigen representation on the sperm surface is coded for by the haploid genome. In a broader context, in as much as H-Y incompatibility is now seen to be a system in which graft rejection may not occur despite an immune response (antibody production), it provides a parallel with the growth of antigenic tumors in hosts that produce antibody and therefore has relevance to cancer as well as to transplantation biology.